In a typical cellular network, also referred to as a wireless communication system, User Equipments (UEs), communicate via a Radio Access Network (RAN) to one or more Core Networks (CNs).
A user equipment is a mobile terminal by which a subscriber can access services offered by an operator's core network. The user equipments may be for example communication devices such as mobile telephones, cellular telephones, laptops or tablet computers, sometimes referred to as surf plates, with wireless capability. The user equipments may be portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another mobile station or a server.
User equipments are enabled to communicate wirelessly in the cellular network. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between the user equipment and a server via the radio access network and possibly one or more core networks, comprised within the cellular network.
The cellular network covers a geographical area which is divided into cell areas. Each cell area is served by a base station, e.g. a Radio Base Station, RBS, which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used.
The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also on cell size.
A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
In some radio access networks, several base stations may be connected, e.g. by landlines or microwave, to a radio network controller, e.g. a Radio Network Controller (RNC) in Universal Mobile Telecommunications System (UMTS), and/or to each other. The radio network controller, also sometimes termed a Base Station Controller (BSC) e.g. in GSM, may supervise and coordinate various activities of the plural base stations connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Spécial Mobile).
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or eNBs, may be directly connected to one or more core networks.
UMTS is a third generation, 3G, mobile communication system, which evolved from the second generation, 2G, mobile communication system GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipments. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
Cellular communication networks evolve towards higher data rates, together with improved capacity and coverage. In 3GPP, standardization body technologies like GSM, HSPA and LTE have been and are currently developed.
To provide for mobility in the cellular network, the cellular network must perform several mobility functions such as handovers when the user equipment moves from one cell to another. A handover means that there is a change of serving cell for the user equipment from a so called source cell to a so called target cell. There are mechanisms in the cellular network to identify which cells are candidate target cells for handover. Typically, the user equipment regularly performs measurements to monitor which cells provide coverage at its current location. The measurement result is sent to a serving base station of the source cell in so called measurement reports. These measurement reports may be used to trigger a handover to the target cell in due time before the user equipment moves out of coverage from the source cell.
If the handover is triggered too early, the user equipment may not be able to connect to the target cell and there is a high probability for oscillating handovers.
If the handover is triggered too late the serving base station of the source cell may not receive the measurement report used for handover triggering, or the user equipment may not be able to receive a handover command from the serving base station of the source cell. Due to this, the handover may not be performed which may eventually lead to the user equipment moving out of coverage from the source cell, detecting a radio link failure, and losing its radio link connection to the cellular network, causing for example an ongoing call or a download to terminate prematurely.
In heterogeneous networks, high power base stations and low power base stations using the same frequency are deployed in the same area such that their coverage, or cells, in the cellular network overlap. Particularly in such environments many handovers may occur.
In view of the above, there is a need to improve robustness of mobility functions in a cellular network, in particular robustness of handovers.